Jonathan's blog

I’ve been practising traditional silver-based black & white photography for a couple of years but today it occurred to me that I don’t really know what is going on with the various chemicals. It’s just a process of remembering which bottle is which. I had a vague idea of what was going on, but I decided to look it up – and summarise it here.

Exposing

The light-sensitive film (or paper) contains crystals of silver halide, which is light sensitive. At this point, the film is opaque grey. When light hits the film, the silver halide crystal splits into a silver ion and a bromine atom.

Ag⁺Br^- (crystal) + hν (radiation) → Ag⁺ + Br + e^-

Then, the silver ion recombines with the free electron to give an atom of metallic silver.

Ag⁺ + e^- → Ag⁰

After exposure, there is an image on the film made from a tiny quantity of metallic silver. This is known as the latent image. It would be invisible to the eye and the film is still dull and opaque. For an individual grain of the silver halide emulsion to count as “exposed”, at least two photons must have interacted with it, to form small silver crystals consisting of two or more silver atoms.

Development

The purpose of developer is to amplify the latent image. The chemical composition of developer varies and is complicated so I won’t go into it here – other than to say that it promotes silver crystal growth where the small silver crystals already exist.

After development, the latent image has been converted to an actual image, made of metallic silver crystals. It appears black, although the film itself is still opaque.

Stopping

Even after taking the film out of the developing solution, it continues to develop (your hands are still wet after taking them out of the sink, right?) so a stop bath is used to halt development. Developing requires an alkaline environment to work, so stop bath is simply a weak acid – usually acetic acid.

The stop bath causes no other changes to the film.

Fixing

Although we have now developed the film and ended up with a black image in metallic silver, the areas of the film that were not exposed to light are still opaque, and still sensitive to light. Bathing the film in fixer dissolves the unexposed silver halide, leaving a near-transparent film backing that is not sensitive to light. At this stage, you can take the film out of the developing tank and look at it in daylight.

Toning

So far, we have ended up with either a film or a print which has an image made from metallic silver. If there’s one thing we know about silver, it’s that it tarnishes. Depending on the storage conditions, silver prints may degrade with time. Toning the image serves two purposes: it improves the longevity of the image, and it can produce the colourful sepia effects.

Various toners exist, but they all work in the same way. They react with the silver to produce silver salts, such as silver sulphide which is more stable then pure silver. It is also slightly brown in colour, hence the sepia tone.

References

• http://www.cheresources.com/photochem.shtml
• http://en.wikipedia.org/wiki/Photographic_developer
• http://en.wikipedia.org/wiki/Photographic_print_toning

When I first started using the Bristol – Bath cycle path I was frustrated by the lack of decent maps. Most of the ones I’ve come across that show the exits of the bike path are schematics, like maps of the London tube. It can be pretty hard to figure out how that ties into a real map, for planning real-life journeys that use roads and off-road paths.

So I spent an hour using the schematic at the link above, Google aerial photos (which show the path), an Ordnance Survey map of Bristol and Bath, and a black pen. I marked in the route of the cycle path and all the exits from the path onto public roads. This map is now on my wall at home and it’s a great reference.

Unfortunately it’s a little too big for my scanner so I’ve had to settle with taking a photo. Unfortunately the problem with photos is that my head casts a shadow, but never mind. Hopefully this will prove useful to someone. Click for a bigger version of the map.